Cellulose ethers are somewhat superior to polyacrylate thickeners such as Carbopol in the viscosity stability of an aqueous solution thereof with an inorganic or organic metal salt. However, cellulose ethers had disadvantages such that their thickening effects were poorer than those of polyacrylate thickeners at the same concentration, and that the viscosity of a hydraulic composition containing cellulose ether as the thickener or dispersant remarkably changed with the temperature.
JP-A 55-110103 and JP-A 56-801 disclose that hydrophobic nonionic cellulose derivatives prepared by partially introducing C.sub.10 -C.sub.24 long-chain alkyl groups into nonionic water-soluble cellulose ethers exhibit relatively high thickening effects even when used in small amounts. Further, attempts to apply such alkylated cellulose derivatives to pharmaceutical preparations for external use, cosmetics and so on are disclosed in JP-A 3-12401, JP-A 3-141210, JP-A 3-141214 and JP-A 3-218316. However, these alkylated cellulose derivatives had problems that they were poor in water solubility, so that it took a long time to dissolve the derivatives homogeneously and that the viscosity of the resulting aqueous solution significantly changed with of time, though they were superior to the above cellulose ethers in thickening effects.
As described above, the cellulose ethers and alkylated cellulose derivatives according to the prior art could not satisfy all of the requirements for the ideal thickener for building materials and so on, i.e., easy dissolution, excellent thickening effect, high dispersion stabilizing effect, no damage to the flowability of building materials, low dependence of the viscosity on coexistent metal salts, surfactants or additives, temperature or pH change, excellent an timicrobial properties, and so on.
JP-A 61-256957 discloses an attempt at imparting plasticity to a mixture of a cementitious material with water by the addition of a water-soluble polymer such as methylcellulose, hydroxymethylcellulose and carboxy-methylcellulose to thereby inhibit the die pressure from lowering and to thereby improve the shape retention of an extrudate. However, the use of such a polymer was not sufficiently effective in inhibiting the die pressure from lowering or in improving the shape retention, and the resulting extruded plate was poor in surface smoothness and appearance. Thus, a further improvement has been expected.